A demand for high-speed promotion for a transmission speed, and an improvement on transmission precision in information communication apparatuses, and semiconductor element testing/inspecting instruments or the like used in the information communication apparatuses has increased with the progress of a recent advanced information-oriented society. For this reason, the high-speed promotion for the transmission speed and the improvement on the transmission precision have been demanded even for a coaxial cable and a coaxial code which have been applied to these apparatuses and instruments.
A relative permittivity of an insulator, and outer diameters of an internal conductor and the insulator participate in transmission characteristics of the coaxial cable. The- transmission characteristics are improved as a value of the relative permittivity is smaller. Also, a ratio of the outer diameter of the internal conductor to the outer diameter of the insulator, and dispersion of the outer diameters of the internal conductor and the insulator largely participate in the transmission characteristics of the coaxial cable. In particular, with respect to a characteristic impedance and an electrostatic capacity, it is ideal that the relative permittivity of the insulator is small, its dispersion is less, dispersion of the outer diameter of the internal conductor and dispersion of the outer diameter (an inner diameter of a shielding layer) of the insulator are less, and the internal conductor and the insulator are formed to have a righter circle cylindrical shapes, respectively.
With regard to a high-precision foamed coaxial cable in which variation in characteristic impedance is reduced, a coaxial cable, for example, described in patent literary document 1 has been known.
Patent literary document 1 discloses a high-precision foamed coaxial cable including an internal conductor constructed by stranding a plurality of conductor wires, a foamed insulator, having a low permittivity, constructed by winding a porous tape around an outer circumference of the internal conductor, an external conductor constructed by braiding a large number of thin conductor wires around an outer circumference of the foamed insulator, and a sheath, made of a resin, having heat resistance, the sheath being formed around an outer circumference of the external conductor, in which precision of an outside diameter size of the internal conductor is set at 4/1000 mm or less, precision of an outside diameter size of the foamed insulator is set at ±0.02 mm, the foamed insulator is formed to have a right circle external shape, precision of an outside diameter size of the external conductor is set at ±2% of a central value of the outside diameter, the external conductor is formed to have a right circle shape, and precision of a characteristic impedance between the internal conductor and the external conductor between which the foamed insulator is interposed is set at ±1 Ω.
According to the high-precision foamed coaxial cable described in patent literary document 1, the irregularities of the external shapes, and the dispersion of the outside diameters in the internal conductor, the insulator, the external conductor and the like constituting the high-precision foamed coaxial cable can be reduced to enhance the precision of the outside diameter sizes, the members can be formed to have the right circle shapes, respectively, and the variation in characteristic impedance can be reduced.
Patent literary document 1: Patent Application Laid-Open No. 2003-234026